1. Field of the Invention
The present invention relates to a gas sensor for detecting a gas such as oxygen in a gas to be measured, which can be the exhaust gas of an internal combustion engine, and a method for manufacturing the gas sensor.
2. Description of the Related Art
A gas sensor, which has a bottomed cylindrical shape having its leading end closed and detecting electrodes on its inner and outer faces, is known for use in detecting a specific gas component in a gas to be measured, such as a gas sensor for detecting oxygen or the like in the exhaust gas of an internal combustion engine. In this gas sensor, the gas concentration is detected by introducing atmospheric air as a reference gas to the inner side of the gas detecting element, and by contacting the outer side with the gas to be measured. An electromotive force, developed according to the difference in gas concentration between the inside and outside of the detecting element, is measured.
In this known gas sensor, moreover, a metallic housing (or a metal shell) encloses the gas detecting element so as to support the gas detecting element at a predetermined position. A gas sensor is also known in which a powder layer is arranged between the gas detecting element and the metallic housing so as to maintain gas-tightness therebetween (as disclosed by JP-A-2001-281209, for example).
3. Problems to be Solved by the Invention
However, the gas sensors of the prior art thus far described have been found to have the following problems. Specifically, the powder layer absorbs moisture, if exposed to a humid environment (i.e., high humidity), so that there is a decrease in electrical resistance between the metallic housing and the outer electrode of the gas detecting element. As a result, an electric current flows therebetween to generate output noise, and precise gas concentration detection may be impaired.
The present invention has been conceived to solve the aforementioned problems. Therefore, an object of the present invention is to provide a gas sensor capable of maintaining requisite insulation properties even in the case where a powder layer subjected to a humid environment may absorb moisture, and to a method for manufacturing the gas sensor.